1. Field of the Invention
The present invention relates to apparatuses for controlling negative pressure in internal combustion engines. More particularly, the present invention pertains to apparatuses for controlling negative pressure in internal combustion engines that are provided with brake boosters, which use negative pressure to improve braking force.
2. Description of the Related Art
Brake boosters have become widely used in vehicles to decrease the required pressing force of the brake pedal. A typical brake booster uses negative pressure, which is produced in an intake passage downstream of a throttle valve, as a drive source. In other words, negative pressure is communicated to the brake booster through a communicating pipe connected to the downstream side of the throttle valve. Negative pressure corresponding to the pressed amount of the brake pedal acts on a diaphragm, which is incorporated in the brake booster, and increases the braking force.
However, internal combustion engines such as diesel engines do not control the amount of air intake during operation. Thus, it is difficult to produce negative pressure at the downstream side of the throttle valve. In such cases, vacuum pumps are provided to produce negative pressure for the brake booster.
Japanese Unexamined Patent Publication No. 61-21831 describes an apparatus that produces negative pressure for the brake booster when the vacuum pump malfunctions. The apparatus slightly closes the throttle valve to produce negative pressure at the downstream side of the throttle valve. The negative pressure is communicated to the brake booster.
However, the employment of a vacuum pump increases the engine load and degrades the fuel efficiency.
Furthermore, in engines that perform stratified charge combustion, stoichiometric air-fuel mixture is supplied to the vicinity of an ignition plug in a cylinder. The other portions of the cylinder are provided with only air. Hence, the throttle valve is substantially completely opened during normal running conditions. As a result, practically no negative pressure is produced at the downstream side of the throttle valve. This causes the negative pressure communicated to the brake booster to be insufficient.